Here's what I found on unassigned DNA on 23andme, which I'm assuming is the same as unknown:

Ancestry Composition has the ability to assign pieces of your DNA at several geographic scales. This is important because DNA varies in where it’s found in the world. One piece of DNA might be found just in, say, Finnish people, so we can with some confidence call this DNA Finnish. Another piece of DNA might be found with some frequency in England, Norway and Germany. Instead of choosing arbitrarily from among these regions, Ancestry Composition will label DNA like that “Nonspecific Northern European."

If a piece of DNA is found all over Europe, like in Ireland, Italy, Spain, and Poland, but not outside Europe, Ancestry Composition will label that DNA “Nonspecific European."

Finally, when Ancestry Composition encounters DNA that is very widespread, like a piece of DNA that’s found in Europe, the Near East, and Asia, it will label that DNA “Unassigned.”